Highly sensitive analytical instruments with measuring sensors in a high vacuum (such as mass spectrometers) are nowadays able to measure concentrations of trace substances down to 1 ppb (one billionth of the weight) at a total gas inflow of only 10 nanoliters per second (corresponding to approximately 10 nanograms per second). These are trace substance flows of only 10.sup.-17 grams per second, corresponding to 10.sup.5 molecules per second for a substance with a molecular weight of 60 Daltons (one Dalton is equal to one atomic mass unit).
If a surface coming into contact with the inflowing gas is active in adsorbing one of the trace substances to be analyzed, it can very easily completely prevent the flow of this trace substance into the measuring instrument for a long time. In comparison, a single square millimeter of an active surface can adsorb 1 nanogram (10.sup.-9 grams) of a substance with a molecular weight of approximately 300 Daltons before it is saturated with a monomolecular layer.
Chromatographic capillary columns, which have an excellent low level of adsorption, are nowadays almost exclusively used for inflow of the substance to the mass spectrometer. The columns are heated in order to keep the substances vaporous and avoid condensation. High-temperature-resistant chromatographic phases capable of being heated up to 400.degree. C. are already known.
It is advantageous to keep analytical high-vacuum instruments, such as mass spectrometers, permanently at a high vacuum in order not to have to constantly reproduce clean measuring conditions by means of lengthy baking processes when the measuring systems are returned to use. Accordingly, high-vacuum analyzers, which do not operate with permanently connected capillary columns and are kept at a vacuum round the clock by active pumping, require an ultrahigh vacuum valve (UHV valve) in order to admit substances for testing. This particularly concerns mass spectrometers for mobile use evacuated with ion getter pumps and transported at a vacuum in a sealed state.
The valve used with such systems must be ultrahigh-vacuum-tight and able to withstand baking periods up to 400.degree. C. without any limitation of function. The normal operational temperature during opening times is 250.degree. C. in order to avoid condensation of the substances admitted.
Inlet valves for substance mixtures to be analytically examined for trace materials must meet the requirements stated above concerning freedom of adsorption. On the other hand, inlet valves need to have elastic wall pieces, such as metal membranes or metal bellows, in order to be able to move a valve piston into the inlet path by means of an external force.
These elastic wall pieces, however, constitute a problem for surface adsorption because the choice of material for the surface is limited by the elasticity requirements. Since they have to accommodate a motion stroke, they generally have an unfavorably large surface area. In addition, the surface of metal membranes or metal bellows frequently contains rolled-in organic materials. These materials are given a diffusion thrust by each elastic movement and am released in minute traces, thus, a chemical background is constantly present. This behavior cannot be eliminated either by baking or by cleansing surface treatments.
It is the task of the invention to produce a valve in which disruptive adsorption and outgassing into the substance flow entering the analytical instrument are restricted to a minimum.